CVE-2026-5751 json
justhtml before 1.14.0 Mutation XSS via custom sanitization policies
published 2026-04-08 7 mo ago modified 2026-10-12 28 d ago also GHSA-r758-8hxw-4845 , EUVD-2026-64654
- cvss
- 6.1
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.3%
- EPSS 0.3% chance of exploitation in 30 days, percentile 18%
- public exploits
- 0
- none known
Summary
justhtml versions 1.13.0 and earlier contain a parser-differential / mutation cross-site scripting (mXSS) vulnerability when using a custom SanitizationPolicy that preserves foreign namespaces (e.g., drop_foreign_namespaces=False with allowlisted SVG/MathML elements or raw-text containers such as <style>). Specially crafted input can sanitize into markup that appears safe but becomes unsafe when re-parsed by a browser or another HTML parser, allowing markup injection. The default safe configuration (sanitize=True) is not affected. Fixed in 1.14.0.
Patch / fix
- pypi justhtml 1.14.0
- EmilStenstrom justhtml 1.14.0
CISA SSVC
- Exploitation
- none
- Automatable
- no
- Technical impact
- partial
- Assessed
- 2 mo ago
Exploit prediction
0.26% chance of exploitation in the next 30 days, higher than 18% of all scored CVEs (FIRST EPSS, 2026-11-08).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 4.0 | 5.1 | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N |
| CNA | 3.1 | 6.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N |
| CNA | 4.0 | 5.1 | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| pypi | justhtml | >=1.13.0, <1.14.0 |
1.14.0 |
Affected products
- EmilStenstrom justhtml
Classification
- Weakness
-
- CWE-79 Improper Neutralization of Input During Web Page Generation
References
-
github.com
/EmilStenstrom/justhtml/commit/8adc4dfe31723dc522f44338f9bd98381c92b076
web
-
github.com
/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845
vendor-advisory, web
-
vulncheck.com
/advisories/justhtml-before-mutation-xss-via-custom-sanitization-policies
third-party-advisory
-
github.com
/EmilStenstrom/justhtml
package
-
github.com
/EmilStenstrom/justhtml/releases/tag/v1.14.0
web
Sources
- CVE List CVE-2026-5751
- GitHub Advisory GHSA-r758-8hxw-4845
- NVD CVE-2026-5751
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